Electromagnetic device for converting mechinal vibrational energy into electrical energy, and manufacture thereof

ABSTRACT

An electromagnetic generator comprising a multilayer assembly of a first layer carrying at least one magnet, a second layer carrying at least one coil, and a third layer carrying at least one magnet, the at least one magnet of the first and third layers being configured to define therebetween a region of magnetic flux in which the at least one coil is disposed, at least one of the layers being shaped to define a respective displaceable portion thereof which is displaceable by vibration of the electromagnetic generator thereby to cause relative movement between the coil and the magnets and generate an electrical current in the coil.

The present invention relates to an electromagnetic generator forconverting mechanical vibrational energy into electrical energy, and toa method of manufacturing such an electromagnetic generator. The presentinvention also relates to a magnetic core for an electromagneticgenerator for converting mechanical vibrational energy into electricalenergy, to such an electromagnetic generator including such a core, andto a method of producing a magnetic core for an electromagneticgenerator. In particular, the present invention relates to such a devicewhich is a miniature generator capable of converting ambient vibrationenergy into electrical energy for use, for example, in poweringintelligent sensor systems. Such a system can be used in inaccessibleareas where wires cannot be practically attached to provide power ortransmit sensor data.

There is currently an increasing level of research activity in the areaof alternative power sources for micro electrical mechanical systems(MEMS) devices, such devices being described in the art as being usedfor ‘energy harvesting’ and as ‘parasitic power sources’. Such powersources are currently being investigated for powering wireless sensors.

It is known to use an electromagnetic generator for harvesting usefulelectrical power from ambient vibrations. A typical magnet-coilgenerator consists of a spring-mass combination attached to a magnet orcoil in such a manner that when the system vibrates, a coil cuts throughthe flux formed by a magnetic core. The mass which is moved whenvibrated is mounted on a cantilever beam. The beam can either beconnected to the magnetic core, with the coil fixed relative to anenclosure for the device, or vice versa.

In a paper entitled “Design and fabrication of a new vibration basedelectromechanical power generator”, by Glynne-Jones et al, published inSensors and Actuators A92, 2001, pp. 335-342, an electromechanicalgenerator was disclosed consisting of a cantilever beam supported by ahousing. A mass on the beam was made up of two magnets mounted on akeeper to form a C-shaped core. A coil was placed in the air-gap betweenthe magnets at right angles to the direction of movement of the mass onthe cantilever beam. While this prior disclosure by some of the presentinventors produced a useful electromechanical generator, there is stilla need to enhance the efficiency of the conversion of mechanicalvibration energy into electrical energy, and thereby into usefulelectrical power.

In a later paper entitled “An electromagnetic, vibration-poweredgenerator for intelligent sensor systems”, by P Glynne-Jones, M J Tudor,S P Beeby, N M White, Department of Electronics and Computer Science,University of Southampton, Southampton, SO17 1BJ, Hampshire, England,which was published at a conference entitled “Eurosensors XV1” held in2002 in Prague, Czech Republic, an improved electromechanical generatorwas disclosed. The electromechanical generator incorporated fourmagnets, which created a magnetic field through a greater proportion ofthe length of each coil winding when compared to double or single magnetdesigns. The magnets and core structure of the device are illustrated inFIG. 1.

A web page “http://www.iee.org/oncomms/pn/measurement/Steve%20Beeby.pdf”is a copy of a presentation entitled “Kinetic energy harvesting forwireless energy systems” by S P Beeby et al, made at the Institute ofElectrical Engineers (IEE) in the United Kingdom at a seminar on“Wheatstone Measurement” held on 11 Dec. 2002. That presentationsimilarly disclosed the structure and use of an electromechanicalgenerator having the magnet, core and coil construction shown in FIG. 1.

For each of these latter two prior disclosures made by some of thepresent inventors, although the disclosed electromechanical generatorhad a good efficiency, there is still a need to improve the design toenhance the efficiency of electrical power generation from mechanicalvibrations, and to provide an improved manufacturing method, inparticular so as to provide a low production cost.

The magnetic core structure, designated generally as 2, in FIG. 1comprises four magnets 4,6,8,10. Each magnet 4,6,8,10 is substantiallyblock shaped having opposed ends of opposite polarity. The four magnets4,6,8,10 are disposed in two magnet pairs, with each pair of magnets4,6;8,10 being assembled with a respective keeper plate 12,14 offerromagnetic material, for example steel. For each pair of magnets4,6;8,10, the end of one magnet (for example magnet 4) having a firstpolarity (for example N for the magnet 4 in FIG. 2) is assembled againstthe respective keeper plate (for example keeper plate 12 in FIG. 2) andan end of opposite polarity (for example S) of the other magnet (magnet6 in FIG. 1) is assembled against the same keeper plate (keeper plate12). The two pairs of magnets 4,6;8,10 are mounted in an opposingmanner, with magnet ends 16,18;20,22 of opposite polarity spaced fromand facing each other, and with the magnetic flux being guided aroundthe two opposed outside edges of the magnetic core 2 by means of the twokeeper plates 12,14, thereby to define a magnetic circuit.

With this arrangement, a single elongate slot 24 is defined between thetwo opposed magnet pairs 4,6 and 8,10 and there are also defined in themagnetic circuit two air gaps 26,28 therein, each air gap 26,28 beingdefined between respective opposed magnet ends 16,18;20,22. As shown inFIG. 1, the coil 30 is disposed in the slot 24. The magnetic circuit ismounted on a cantilever beam (not shown), for example a U-shaped member,with each end of the U-shaped member connected to a respective pair ofmagnets 4,6;8,10. When the electromechanical generator is subject tomechanical vibration, the cantilever beam can correspondingly vibrate,in an up and down direction with respect to the magnetic circuit, asshown by the arrows indicating magnet movement in FIG. 1. This causes anelectrical current to be generated in the coil 30.

For each of these latter two prior disclosures made by some of thepresent inventors, although the disclosed electromechanical generatorhad an improved efficiency as a result of a magnetic field being createdthrough a greater proportion of the length of each coil winding whencompared to double or single magnet designs, there is still a need toimprove the design to enhance the efficiency of electrical powergeneration from mechanical vibrations.

U.S. Pat. No. 6,304,176 in the name of Rockwell Technologies LLCdiscloses a parasitically powered sensing device for monitoring anindustrial system. A tuned transducer converts stray energy emitted bythe system into an electrical potential for consumption by a remotesensing device and/or a wireless communications link. The parasitictransducer may be a piezo-electric crystal element coupled to a tunedmechanical oscillator. Alternatively, the sensing element and transducermay be in the form of a micromechanical system. However, no specificmagnet, core and coil arrangement is disclosed.

The present invention aims to provide to an improved electromagneticdevice for converting mechanical vibrational energy into electricalenergy, and to an improved method for its manufacture.

The present invention also aims to provide to an electromagnetic devicefor converting mechanical vibrational energy into electrical energywhich has a greater energy conversion efficiency than known devices.

The present invention accordingly provides an electromagnetic generatorcomprising a multilayer assembly of a first layer carrying at least onemagnet, a second layer carrying at least one coil, and a third layercarrying at least one magnet, the at least one magnet of the first andthird layers being configured to define therebetween a region ofmagnetic flux in which the at least one coil is disposed, at least oneof the layers being shaped to define a respective displaceable portionthereof which is displaceable by vibration of the electromagneticgenerator thereby to cause relative movement between the coil and themagnets and generate an electrical current in the coil.

The present invention yet further provides a method of manufacturing anelectromagnetic generator, the method comprising the steps of:

(a) forming a first layer carrying at least one magnet, forming a secondlayer carrying at least one coil and forming a third layer carrying atleast one magnet, at least one of the layers being shaped to define arespective displaceable portion thereof which is displaceable byvibration, the displaceable portion carrying either the at least onemagnet of the first and third layers or the at least one coil of thesecond layer; and

(b) assembling together the first, second and third layers to form amultilayer structure in which the magnets of the first and third layersare configured to define therebetween a region of magnetic flux in whichthe at least one coil is disposed, the at least one displaceable portionbeing displaceable by vibration of the multilayer structure thereby tocause relative movement between the coil and the magnets and generate anelectrical current in the coil.

The present invention yet further provides an electromagnetic generatorcomprising at least two magnets and at least one coil disposedtherebetween, the at least two magnets being configured to definetherebetween a region of magnetic flux in which the at least one coil isdisposed whereby relative movement between the coil and the magnetsgenerates an electrical current in the coil, and at least onepiezoelectric region which is adapted to generate additional electricalcurrent by relative movement between the coil and the magnets.

The present invention also provides a magnetic core for anelectromagnetic generator, the magnetic core comprising four magnetsdisposed in two magnet pairs, with each pair of magnets being assembledwith a respective keeper, the two pairs of magnets being mounted in anopposing manner so that a front end of each magnet of one magnet pair isspaced, in a first direction, from and faces a front end of acorresponding magnet of the other magnet pair, the facing front endsbeing of opposite magnetic polarity, thereby to define in the magneticcore a pair of gaps between the front ends of the four magnets, and withrear ends of the magnets of each pair contacting a respective keeper,the magnets of each pair being mutually spaced in a second direction,and wherein the ratio between the width of each magnet in the seconddirection to the height of the magnetic core in the second direction isfrom 0.4 to 0.55.

The present invention further provides an electromagnetic generator, theelectromagnetic generator comprising a magnetic core according to thepresent invention, a coil disposed in the pair of gaps and a vibrationsensitive mount for mounting one of the magnetic core and the coilwhereby vibration of the electromagnetic generator causes relativemovement of the magnetic core and the coil thereby to generate anelectrical current in the coil.

The present invention yet further provides a method of producing amagnetic core for an electromagnetic generator, the magnetic corecomprising four magnets disposed in two magnet pairs, with each pair ofmagnets being assembled with a respective keeper, the two pairs ofmagnets being mounted in an opposing manner so that a front end of eachmagnet of one magnet pair is spaced, in a first direction, from andfaces a front end of a corresponding magnet of the other magnet pair,the facing front ends being of opposite magnetic polarity, thereby todefine in the magnetic core a pair of gaps between the front ends of thefour magnets, and with rear ends of the magnets of each pair contactinga respective keeper, the magnets of each pair being mutually spaced in asecond direction, the method comprising the steps of:

(a) establishing a model for the geometrical parameters of the magneticcore, the parameters including the width of each magnet in the seconddirection (t_(m)), the height of the magnetic core in the seconddirection (l_(c)), the length of each magnet in the first direction(l_(m))and the length of the gap in the first direction (g);

(b) varying the parameters to provide an output value ψ, which isdefined by the equation${\psi = \frac{\int^{airgap}{B^{2}{\mathbb{d}A}}}{{total}\quad{area}\quad{of}\quad{core}}};$

wherein B is the magnet flux density; and

A is the total face area of each magnet pair of the core, the facesdefining the air gaps; and

the total area of the core is the total face area of each magnet pairplus the face area of the gap therebetween.

(c) determining a maximum for the parameter ψ;

(d) determining values of at least the parameters (t_(m)), (l_(c)),(l_(m)) and (g) to provide a range for the parameter v which encompassesthe maximum for the parameter ψ; and

(e) producing the magnetic core having the determined values of theparameters (t_(m)), (l_(c)), (l_(m)) and (g) within a particulartolerance.

The electromechanical generator of the present invention has particularapplication in the provision of electrical power to sensor systems. Inaccordance with the invention, typical application areas for such selfpowered intelligent sensor systems are: inside or on the body (e.g.human, animal); on rotating objects; within liquids such as moltenplastic or setting concrete; structural monitoring such as withinbridges, buildings, aircraft or roads; and environmental monitoring,such as pollution monitoring in fields.

The electromagnetic generator in accordance with the present inventionhas a number of potential uses and applications, particularly whenminiaturised. For example, the electromagnetic generator is reallyuseful in applications where cabling to a unit (e.g a sensor unit)requiring electrical power is difficult and/or expensive and batterypower is insufficient in the life of the unit, weight is important andthere is a significant level of vibration available to harvest powerfrom. In some cases the vibration harvesting technology of theelectromagnetic generator may be used to provide a charging facility toa battery powered system.

In one particularly preferred application, the electromagnetic generatorof the present invention may be incorporated into Health and UsageMonitoring Systems (HUMS) for helicopters and fixed wing aircraft.

HUMS systems monitor vibration and other parameters related tohelicopter (or other aircraft) condition and the number of hours flyingin defined stress conditions. The installation of sensors and retrievingdata from those devices is a major cost issue both at installation andalso during planned maintenance. The benefits of the electromagneticgenerator of the present invention are reduced installation costs andshorter time taken for maintenance. The sensor would be packaged with alocal wire less transmission system and would transmit data to the HUMSmonitoring system. Some advantages of using the electromagneticgenerator powered sensor system is that the complexity of installingwiring on existing or new airframes is avoided and the weight of thecabling is eliminated. Also monitoring sensors can be mounted for shorttrial periods without high installation costs.

In another particularly preferred application, the electromagneticgenerator of the present invention may be incorporated into sensingsystems for railway lines and associated components.

The condition of railway lines and associated components is a matter ofconcern within the UK and probably throughout the world. It is known toprovide sensor systems for sensing for rail condition and also thepresence/absence of vital components. In some situations there isadequate local electrical power for driving a sensor. However, in othersituations local electrical power may be unavailable or inconvenient,particularly for remote or distant sites, in which case there is a needfor a self powered sensor (e.g. a strain sensor) that could telemeterthe output data to a single powered point (one for a large geographicalarea) or via a GPS link. The vibration for the sensor may be provided bythe passage of a train, either directly from the rail line or via acantilever attached to the line. Other railway infrastructure monitoringincludes, for example, strain measurement in rails, ballast conditionand height, and points monitoring.

In a yet further particularly preferred application, the electromagneticgenerator of the present invention may be incorporated into a vehiclebattery charger system, for example for lorry or truck trailers trackingbattery recharging.

Articulated trailers need to be tracked for logistics applications. Inthis application the trailer is only powered when the trailer isconnected to a tractor unit. Even then there may be no power availablefor a retrofitted tracking system. If the system is powered by a batteryit would be an advantage to have an independent charging system thatwould charge the battery whilst the system was being towed. The chargingsystem may incorporate the electromagnetic generator of the presentinvention. The battery would then be able to power the tracking systemwhile the trailer was stationary and disconnected from the tractor unit.

In a still further particularly preferred application, theelectromagnetic generator of the present invention may be incorporatedinto a mobile telecommunications equipment, for example militaryback-pack telecommunications equipment (e.g. Bowman), which is poweredby a battery. Batteries contribute to a significant proportion of theoverall weight of the equipment carried in the field. Clearly duringfield operation the equipment is subjected to considerable vibration.These vibrations could be transformed by the electromagnetic generatorof the present invention into electrical power that is suitablyconditioned for use in recharging the battery packs. The electromagneticgenerator of the present invention can reduce the weight and maintainthe power available of the telecommunications equipment, therebyproviding real benefits to the user.

In other preferred applications, the electromagnetic generator of thepresent invention may be incorporated into a conditioning monitoringsystem which is increasingly used in a very wide field for many types ofequipment. For example, the electromagnetic generator of the presentinvention may be used to power a vibration condition monitoring sensoron any type of equipment.

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side elevation of a configuration for the magnets,core and coil of a known electromagnetic device for convertingmechanical vibrational energy into electrical energy;

FIG. 2 is a schematic perspective view of an electromechanical generatorin accordance with a first embodiment of the present invention;

FIG. 3 is a schematic perspective exploded view of the electromechanicalgenerator of FIG. 2;

FIG. 4 is a schematic plan view of the structural arrangement of thecoil and cantilever in the electromechanical generator of FIG. 2;

FIG. 5 is a schematic perspective view of an electromechanical generatorin accordance with a further embodiment of the present invention;

FIG. 6 is a cross-section through an electromechanical generator inaccordance with another embodiment of the present invention;

FIG. 7 is a schematic side elevation of a magnetic core structure for anelectromechanical generator in accordance with a further embodiment ofthe present invention;

FIG. 8 shows a finite element model used in the present invention todetermine the properties of a magnetic core of an electromechanicalgenerator;

FIG. 9, FIG. 10 and FIG. 11 show typical magnetic flux patterns ofmagnetic cores which have been calculated in accordance with the modelused in the invention for different extremes of geometricalconfiguration of the magnetic core; and

FIG. 12 shows a magnetic field pattern for a magnetic core of anelectromechanical generator in accordance with a preferred embodiment ofthe present invention

A magnet-coil generator in accordance with the present inventionconsists of a spring-mass combination attached to a magnet or a coil insuch a manner that when the system resonates, the coil cuts through theflux formed by the magnet as a result of relative movement between themagnet and the coil. The spring-mass is typically a cantilever beamwhich can either be connected to the magnet, with the coil fixedrelative to an enclosure for the magnetic coil generator, or vice versa.The two possible geometries are referred to as a moving magnet geometryor a moving coil geometry.

Referring to FIGS. 2 to 4, an electromagnetic generator, designatedgenerally as 100, in accordance with an embodiment of the presentinvention comprises a multilayer structure 101, in which a number oflayers are sandwiched together. A central layer 102 is provided with acoil 104 and two opposed outer layers 106,108, each being disposed on arespective side of the central layer 102, are each provided on theirinner face 107, 109 with respective magnets 110,112. The magnets 110,112 may alternatively be provided on the respective outer face of theouter layers 106, 108. The magnet faces 114, 116 which are disposedtowards the coil 104 have opposite polarity, so that magnetic fluxextends therebetween. The coil 104 is free to be moved, as a result ofvibration applied to the electromagnetic generator 100, in a cavity 117defined between the two opposed outer layers 106,108. The coil 104 movesin the plane of the central layer 102. The movement direction is shownby the arrows in FIG. 4. The magnets 110,112 are located so that theirmagnetic flux is cut by movement of the coil 104, whereby an electricalcurrent is generated in the coil 104. Such a current is fed along bondwires 118 connected to the coil 104.

As shown more clearly in FIG. 3 , in the electromechanical generator ofthe invention the magnets poles facing each other on opposite sides ofthe coil 104 have opposite polarity (i.e. one is north N and one issouth S). If there are two magnets on each opposite side of the coil104, and thus four magnets in total, each magnet pair face each otherwith poles of opposite polarity (i.e. one is north N and one is southS), and on each side of the coil there is one magnet face of eachpolarity (i.e. one is north N and one is south S).

The multilayer structure 101 typically has a thickness of from 1.5 to 2mm, and typically has dimensions of about 5 mm by 5 mm in plan.

Each of the outer layers 106, 108 typically has a thickness of from 0.5to 0.75 mm. The outer layers are most typically composed of borosilicateglass (such as that available in commerce under the Trade Mark“Pyrex”®), but alternatively may be composed of other materialsavailable commercially in wafer form, for example silicon or galliumarsenide, or alternatively may be composed of materials which arecompatible with thick film processing, for example alumina, steel ormetallic alloys, such as for example Inconel.

The outer layers 106, 108 are most preferably produced using stepsemployed for wafer technology. Thus a single wafer is processed by aseries of steps which commonly produce a plurality (typically hundreds)of outer layers all integral on the common wafer. As described below,two wafers incorporating the outer layers are bonded to opposed sides ofa central wafer correspondingly incorporating a plurality of the centrallayers 102 the and then, after bonding of the wafers together, thecomposite three wafer assembly is cut (or diced) to form a plurality ofseparate and individual bonded assemblies, each including two outerlayers 106, 108 with a central layer 102 sandwiched therebetween.

Each of the outer layers 106, 108 is provided thereon with a pair ofspaced magnets 120,122; 124,126 which have been applied to a surface ofthe respective wafer to form the respective outer layers 106, 108.Typically, the magnets 120,122; 124,126 are thick film magnets whichhave been produced by screen-printing an ink including magnetic ormagnetisable material onto the surface of the wafer which is ultimatelyto form the respective outer layer 106, 108. Alternatively, the magnets120,122; 124,126 may be fabricated by other methods, such as thin filmdeposition or electroplating. After the magnetic layers have beenformed, the magnets are polarised to the correct polarity.

In an alternative embodiment, only a single magnet is provided on eachof the two outer layers 106, 108.

In a her alternative embodiment, the magnetic layers may be provided onthe outer surface of the outer layers 106, 108 rather than in the cavity117 defined between the outer layers 106, 108.

In a yet further embodiment, instead of forming magnetic layers on thesurface of the outer layers 106, 108, as shown in FIG. 5 recesses 128are etched in the outer layers 106, 108 and bulk pre-formed andpre-polarised magnets 130 are inserted into the recesses 128 and fixedtherein, for example by adhesive. In this embodiment, there are twomagnets 130 on each of the outer layers 106, 108 and the magnets are onthe outer surface of the respective layers 106, 108. However, therecould be only one magnet 130 on each of the outer layers 106, 108.

The bulk magnets 130 may however be disposed so as to be located eitherin the cavity 117 between the outer wafers or on the outside of themultilayer structure. The use of such bulk magnets can provide highermagnetic flux than achievable with magnetic layers and also avoids anadditional polarising step which is needed to form magnets of therequired polarity from the applied magnetic layers. The bulk magnetstypically have dimensions of 1 mm by 1 mm in plan and a thickness ofabout 0.75 mm.

When bulk magnets are employed, to improve the degree of coupling, it isdesirable to choose a type of magnet that will produce a strong fluxdensity. Rare earth magnets are ideal for this application, and offer upto 5 times the magnetic energy density of conventional Alnico magnets.Neodymium Iron Boron (NdFeB) magnets have the most powerful magneticproperties per cubic cm known at this time, and can operate at up to120° C. If higher temperature operation is required, the less powerfulSamarium Cobalt can be used, with a working temperature of up to 250° C.

The central layer 102 is separately fabricated using wafer technology,as used for the outer layers 106, 108, and is also composed of amaterial which is commercially available in wafer form, for exampleborosilicate glass, silicon or gallium arsenide, or of a materialcompatible with thick form processing, for example alumina, steel ormetallic alloys such as Inconel. Most typically, the central layer 102is composed of silicon, because silicon has good mechanical propertiesfor use in forming a cantilever beam as described below.

The central layer 102 comprises a peripheral frame 134 surrounding acentral body 136, referred to as a paddle, which is attached to theperipheral frame 134 by a single beam element 138, thereby forming acantilever. The paddle 136 is surrounded, apart from the beam element138, by a cutout 140 extending through the thickness of the centrallayer 102. The beam element 138 is dimensioned so that it is flexible,thereby allowing movement of the paddle 136 within the cutout 140, inthe plane of the central layer 102 but stiff with respect to movementout of the plane of the central layer 102. If desired, additionalstiffness and control of the movement of the central paddle 136 may beincorporated by including additional beam elements between the centralpaddle 136 and the surrounding peripheral frame 134. The spacing “d”between the sides 142 of the central paddle 136 and the opposed facingsides 144 of the surrounding peripheral frame 134 is selected so as tobe slightly larger than a preset maximum working amplitude of vibrationof the central paddle 136, but so that the relatively thin beam element138 does not break or otherwise become damaged if the central paddle 136inadvertently hits the surrounding peripheral frame 134.

The structure of the central layer 102 is achieved by etching a wafer,for example by deep reactive ion etching, so as to form an openingdefining the cutout 140 between the peripheral frame 134 and the centralpaddle 136.

The central paddle 136 incorporates one or more integrated coils 146 onone or both faces 148 thereof. Most preferably, the or each coil 146 isproduced on a respective surface of the central paddle 136 so as to beintegrally formed on the surface, for example by thick film printing orelectrochemical deposition. The or each coil 146 is dimensioned so asmaximally to cut the magnetic flux produced by the magnets 110, 112 whenthe centre paddle 136 is vibrated laterally at its full amplitude in theultimate electromagnetic generator 100. Thereafter, electricalconnections 150 to the or each coil 146 are formed on the wafer so as topermit bond wires 118 subsequently to be attached to the coil 146. Theelectrical connections may be produced by a number of well-knownmethods, such as indiffusion or metallisation.

In a particularly preferred embodiment, at least one region ofpiezoelectric material is additionally provided in the electromagneticgenerator so that the device generates electrical current as a result ofexternal vibration not only by movement of a coil through magnetic fluxbut also by the application of stress to the piezoelectric material.

In accordance with an embodiment incorporating this aspect of thepresent invention, at least one area of active piezoelectric material160 is printed onto one or both of the outer layers 106, 108 waferswhich carry the magnets 110, 112 in the region where the paddle 136 ofthe central layer 102 would impact in the event of movement of thepaddle 136 beyond a preset maximum amplitude. In addition, oralternatively, at least one area 162 of active piezoelectric material ispreferably printed on any additional strained part of the material, suchas the supporting beam element 138 for the paddle 136. Such additionalareas of active piezoelectric material are connected electrically (bymeans not shown) to the bond wires 118. The provision of such additionalpiezoelectric material permits the generation of additional usefulelectrical energy harvested from the piezoelectric effect in addition tothat harvested by the movement of the coil in the magnetic fluxgenerated by the magnets.

The three layers, comprising the outer layers 106, 108 and the centrallayer 102, are assembled together to form a multilayer assembly. Theassembly process may assemble all three layers together eithersimultaneously or consecutively. The layers are typically bondedtogether using a wafer bonding technique such as silicon fusion bondingor electrostatic bonding.

If necessary, so as to ensure an appropriate clearance between thepaddle 136 and the magnets 110, 112, to enable unimpeded movement of thepaddle 136 relative to the magnets 110, 112, a peripheral spacer (shownas 170 and 172 in the embodiment of FIG. 6) may be provided on each sideof the central layer 102 between the central layer 102 and therespective outer layer 106, 108. Alternatively, the thickness of thepaddle 136 may be reduced, for example by etching, during its formationin order to accommodate the magnets 110, 112, thus avoiding the need fora peripheral spacer.

As shown in FIG. 6, which is a cross-section along line X-X of FIG. 5but of a different embodiment, the coil comprises a wire coil 146, whichis incorporated into the electromagnetic generator, instead of formingthe coil 146 on the surface of the central layer 102. In thisembodiment, a wire wound coil 146 is provided on one of the two oppositefaces 147, 149 of the paddle 136. Thus a pre-formed wire coil may beattached, for example by adhesive, on to one surface of the paddle 136.If desired, the coil 146 may be disposed in a respective etched recess151 formed in the surface of the paddle 136 in order to accommodate thewound coil 146. This particular embodiment offers the advantage of ahigh level of performance as a result of the provision of a wound coil,coupled with the excellent mechanical properties of silicon which formsthe paddle 136 together with the integral cantilever beam element 138.If desired, two coils 146 may be provided, one on each face 147, 149 ofthe paddle 136.

In accordance with the preferred method of the invention, waferprocessing is employed to produce the outer layers 106, 108 and thecentral layer 102. In other words, an array of a plurality (typicallyhundreds) of outer layers 106, 108 are respectively simultaneouslyproduced on two first wafers, and an array of a plurality(correspondingly, typically hundreds) of central layers 102 are producedon another single second wafer, and then those wafers (two first wafersand one second wafer) are bonded together, for example by electrostaticor silicon fusion bonding, to form a unitary multilayer structure. Thismultilayer structure is then cut up into a plurality of individual threelayer devices 101, and then bonding wires 118 are attached to eachdevice 101. Each device 101 is then encapsulated, if desired, into arespective housing (not shown).

Although the electromagnetic generator of the illustrated embodimentincorporates a three layer structure, in accordance with other aspectsof the invention the electromagnetic generator may incorporate a stackor array of plural three layer units in order to achieve higher outputpower.

In the illustrated embodiment, the electromagnetic generator includesfour magnets, two magnets being disposed on each side of the coil. Thisconfiguration creates a magnetic field through a greater proportion ofthe length of each winding as compared to other configurations employingdouble magnet designs (one magnet on each side of the coil). Thistherefore reduces the resistive losses in the coil windings byshortening the coil for a given degree of electromagnetic coupling.However, the present invention may employ only two magnets, one on eachside of the coil.

Also, it is optional for a keeper element to be provided for each magnetpair on each side of the coil, the keeper contacting both of theopposite polarity faces of the two magnets of each pair.

The coil is characterised by the proportion of the coil that passesthrough the magnetic field, the number of turns in the coil, and itsseries resistance. Second-order effects such as coil inductance canoften be ignored due to the low frequency of many applications. Asdisclosed above, two types of coil may be used in the present invention:wound coils, and printed coils.

A printed coil can be formed by screen-printing layers of conductivematerials and insulators onto a substrate in much the same manner asprinted circuit boards (PCBs) are produced. A printed coil can be madevery thin as printed layers will typically be 10 μm thick, making thisapproach particularly attractive for small-scale devices. A printed coilmay also be easier to manufacture as it only involves standardthick-film printing processes, as opposed to a wound coil, which becomesmore difficult to manufacture particularly as the scale decreases. Thedisadvantage of a printed coil is that the small thickness of each layerwill result in a high series resistance for the coil. If windings of alarger thickness than are traditionally available from thick-filmtechnology (e.g. >50 μm) are required a wound coil will be more suitableand economic to manufacture. Printed coils have the added advantage ofalready being attached to a substrate, which may add rigidity to thecoil, and hence decrease the clearance required between the coil and themagnets of the outer layers. Additionally the coil may be formed bylithographic processes such as are those used to define structures on asilicon wafer in the technical field of micro-engineering. Theseprocesses are well known in the prior art and successive layers can bebuilt up by a variety of processes such as sputtering, evaporation orelectroplating and are not limited to deposition on silicon wafers butcan be applied to any wafer like substrate.

For efficient energy conversion, it is desired that the beam elementcarrying the paddle be excited at its resonant frequency. This resonantfrequency is sensitive to beam amplitude and environmental temperature.It is also desired to determine the maximum beam amplitude that shouldbe allowed to prevent damage through over straining the beam material.Preferably, the design includes a vacuum-sealed housing so that a vacuumsurrounds the entire device. The vacuum could be produced within thecavity 117, which includes the cutout 140, during the wafer bondingprocess.

In the illustrated embodiment, although each block-shaped magnet havinga longitudinal direction extending between the ends of opposite polarityof the magnet is shown to have a rectangular transverse cross-section,the cross-section may be varied, for example by providing a circularcross-section.

In an alternative embodiment, the coil is in a fixed position and themagnets are adapted for movement relative to the coil as a result ofmechanical vibration imparted to the electromechanical generator. Thus,the magnets are carried on one or more vibratable paddles and the coilis mounted or provided on a solid layer of the multilayer device.

In accordance with the invention, by employing wafer processing andthick film technology to produce a miniature electromagnetic generator,the device can readily be batch fabricated, thus achieving lowproduction cost. Furthermore, such devices are readily miniaturised, yethave high reliability as a result of using known production steps whichare readily controllable.

Yet further, in one particularly preferred aspect of the presentinvention, by combining electromagnetic and piezoelectric harvesting ofelectrical energy from a common input of vibrational energy, this canyield a very efficient device.

Referring to FIG. 7, there is shown a magnetic core structure for anelectromechanical generator in accordance with another embodiment of thepresent invention.

The magnetic core structure, designated generally as 202, in FIG. 7 hassubstantially the same general structural configuration as the knownmagnetic core structure of FIG. 1. Thus the magnetic core structure 202comprises four magnets 204,206,208,210. Each magnet 204,206,208,210 issubstantially block shaped having opposed ends of opposite polarity. Thefour magnets 204,206,208,210 are disposed in two magnet pairs, with eachpair of magnets 204,206,208,210 being assembled with a respective keeperplate 212,214 of ferromagnetic material, for example steel. For eachpair of magnets 204,206,208,210, the end of one magnet (for examplemagnet 204) having a first polarity (for example N for the magnet 204 inFIG. 7) is assembled against the respective keeper plate (for examplekeeper plate 212 in FIG. 7) and an end of opposite polarity (for exampleS) of the other magnet (magnet 206 in FIG. 7) is assembled against thesame keeper plate (keeper plate 212). The two pairs of magnets204,206,208,210 are mounted in an opposing manner, with magnet ends216,218;220,222 of opposite polarity spaced from and facing each other,and with the magnetic flux being guided around the two opposed outsideedges of the magnetic core 202 by means of the two keeper plates212,214, thereby to define a magnetic circuit.

With this arrangement, a single elongate slot 224 is defined between thetwo opposed magnet pairs 204,206 and 208,210 and there are also definedin the magnetic circuit two air gaps 226,228 therein, each air gap226,228 being defined between respective opposed magnet ends216,218;220,222. As shown in FIG. 1, the coil is disposed in the slot224. The magnetic circuit is mounted on a cantilever beam, for example aU-shaped member, with each end of the U-shaped number connected to arespective pair of magnets 204,206,208,210. When the electromechanicalgenerator is subject to mechanical vibration, the cantilever beam cancorrespondingly vibrate, in an up and down direction with respect to themagnetic circuit shown in FIG. 7, and as shown by the arrows indicatingmagnet movement in FIG. 1. During normal operation, the beam amplitudeis not large enough to cause the coil to leave the air gaps 226,228,between the opposed ends 216,218;220,222 of the magnets. When themagnetic core 202 is in its rest position in the absence of anyvibration, the cantilever beam is in a central position, and both theupper and lower portions 232,234 of each turn of the coil pass throughthe magnetic field generated by the magnetic circuit, as shown in FIG.1.

To improve the degree of coupling, it is desirable to choose a type ofmagnet that will produce a strong flux density. Rare earth magnets areideal for this application, and offer up to 5 times the magnetic energydensity of conventional Alnico magnets. Neodymium Iron Boron (NdFeB)magnets have the most powerful magnetic properties per cubic cm known atthis time, and can operate at up to 120° C. If higher temperatureoperation is required, the less powerful Samarium Cobalt can be used,with a working temperature of up to 250° C.

When the magnetic circuit comprising the magnetic core, and the coilthat resides in the magnetic field created by the core, are arranged asshown in FIG. 1, this configuration, which is based on four magnets,creates a magnetic field through a greater proportion of the length ofeach winding as compared to other configurations employing double orsingle magnet designs. This therefore reduces the resistive losses inthe coil windings by shortening the coil for a given degree ofelectromagnetic coupling.

The coil is characterised by the proportion of the coil that passesthrough the magnetic field, the number of turns in the coil, and itsseries resistance. Second-order effects such as coil inductance canoften be ignored due to the low frequency of many applications. Twotypes of coil may be used in the present invention: wound coils, andprinted coils.

A printed coil can be formed by screen printing layers of conductivematerials and insulators onto a substrate in much the same manner asprinted circuit boards (PCBs) are produced. A printed coil can be madevery thin as printed layers will typically be 10 μm thick, making thisapproach particularly attractive for small-scale devices. A printed coilmay also be easier to manufacture as it only involves standardthick-film printing processes, as opposed to a wound coil, which becomesmore difficult to manufacture particularly as the scale decreases. Thedisadvantage of a printed coil is that the small thickness of each layerwill result in a high series resistance for the coil. If windings of alarger thickness than are traditionally available from thick-filmtechnology (e.g. >50 μm) are required a wound coil will be more suitableand economic to manufacture. Printed coils have the added advantage ofalready being attached to a substrate, which may add rigidity to thecoil, and hence decrease the clearance required between the coil and themagnetic core. Additionally the coil may be formed by lithographicprocesses such as are those used to define structures on a silicon waferin the technical field of microengineering. These processes are wellknown in the prior art and successive layers can be built up by avariety of processes such as sputtering, evaporation or electroplatingand are not limited to deposition on silicon wafers but can be appliedto any wafer like substrate.

In an embodiment of the electromagnetic generator of the presentinvention a hand wound coil was attached to an etched stainless steelcantilever to form an inertial mass. NdFeB magnets were held rigidlywith respect to the cantilever with each magnet pair being in anenclosure of an epoxy resin. An embodiment of such an electromagneticgenerator based around a moving coil between four magnets is capable ofgenerating useful power levels from ambient vibrations. For example,such a device produced an average power of 157 μW, and a peak power of3.9 mW when mounted on the engine block of a car. A typical initial coilvoltage amplitude was 250 mV.

For efficient energy conversion, it is desired that the beam be excitedat its resonant frequency. This resonant frequency is sensitive to beamamplitude, environmental temperature, and small variations in theclamping position. It is also desired to determine the maximum beamamplitude that should be allowed to prevent damage through overstraining the beam material. Preferably, the design includes avacuum-sealed cover so that a vacuum surrounds the beam.

In the illustrated embodiment, although each block-shaped magnet204,206,208,210, having a longitudinal direction extending between theends of opposite plurality of the magnet 204,206,208,210, is shown tohave a rectangular transverse cross-section, the cross-section may bevaried, for example by providing a circular cross-section.

Furthermore, although each keeper plate 212,214 is also shown as being arectangular block, the shape of the keeper plate 212,214 can be varied,for example by having a transverse cross-section which is other thanrectangular and/or a cross-section which varies in cross-sectional area

In an alternative embodiment, the magnetic core 202 is in a fixedposition and the coil is adapted for movement relative to the magneticcore 202 as a result of mechanical vibration imparted to theelectromechanical generator.

Referring to FIG. 7, in accordance with the present invention, thepresent inventors have found that a particular shape and configurationfor the magnetic design provides improved energy conversion efficiencyfor converting mechanical vibrational energy into electrical energy. Asshown in FIG. 7, the magnets 204,206,208,210 have a core length, l_(m),and thickness, t_(m). The ferromagnetic keeper plates 212,214 havelength, l_(c), and thickness, t_(c). The width of the air gaps 226,228between the respective magnet pairs 204,206,208,210 is given by g. Thedepth of all the components, comprising the magnets 204,206,208,210 andthe keeper plates 212,214, is equal, and is given by T. The overallwidth of the magnetic core is given by W and the overall length L of themagnetic core is the same as the length l_(c) of the keeper plates212,214. The total face area A of the two magnets is given by 2t_(m)T,and the total area of the core, which is the total face area of eachmagnet pair plus the face area of the gap therebetween, is given by LT.

The present inventors explored the effect of core geometry on themagnetic field in the air gap by generating a finite element model. Themodel exploited the symmetry of the design, and simulated only a quartersection. The model was a planar one, and ignored any edge effects in thedepth direction. The finite element model for a typical set ofdimensions is shown in FIG. 8, and is annotated to show boundaryconditions.

The ferromagnetic keepers were modelled as having a linear B-Hcharacteristic, with a relative permeability of 5000, which is typicalfor Neodynium Iron Boron magnets. The exact value of the permeability isnot critical, as the reluctance of the large air gaps tends to dominatethe results. Saturation was ignored during the finite element analysis,but the design was checked after the modelling to ensure that saturationdid not occur in the ultimate design model.

FIGS. 9, 10 and 11 show typical magnetic flux patterns which have beencalculated for different extremes of geometrical configuration. Ac shownin FIG. 9, when the magnets are close together, most of the flux linesflow straight across the gap, with little leakage. As the magnets areseparated to the position shown in FIG. 10, some of the flux curlsaround between magnets on the same side of the core. This can bepartially alleviated by increasing the length of the core, as shown inFIG. 11.

A batch computer program was written by the inventors to automaticallyvary the geometrical parameters of the model, and calculate suitableoutput data. Output data included the B-field, and the value of∫^(airgap)B²𝕕Afor each configuration. The value of this integral is proportional tothe magnetic energy stored by the magnetic field in the air gap. Sincethe model was a linear one, the B-field predicted by the model was scaleinvariant. Thus, the parameter t_(m) was fixed during the analysis, andthe parameters g, l_(m), and l_(c) varied as proportions of t_(m).

As a result of this analysis, it was found that the core thickness,t_(c), has little effect on the resulting field pattern (so long as itis sufficiently large) and so the core thickness, t_(c), was set to avalue of 2t_(m). It was found that the effect on a typical configurationof doubling t_(c) is to increase the average magnetic field in the airgap by only 0.3%.

After processing, the simulation yielded a 3-dimensional data setshowing the results for each combination of g, l_(m) and l_(c). For eachdata point the minimum value of t_(c) that would avoid magneticsaturation in the core was determined (assuming a value of B_(sat) of 2Tesla), which in turn permitted the total width of the core, W, to befound for each point.

The present inventors then identified and defined a variable ψ (inTesla²) which relates to the amount of magnetic energy stored per corevolume. ψ was defined as: $\begin{matrix}{\psi = \frac{\int^{airgap}{B^{2}{\mathbb{d}A}}}{{total}\quad{area}\quad{of}\quad{core}}} & \left( {{Equation}\quad 1} \right)\end{matrix}$

wherein B is the magnet flux density; and A is the total face area ofeach magnet pair of the core, the faces defining the air gaps (withreference to FIG. 7 this is given by 2t_(m)T); and

the total area of the core is the total face area of each magnet pairplus the face area of the gap therebetween (with reference to FIG. 7this is given by LT).

The present inventors then identified and defined an equation for theparameter P_(L), which is the useful electrical power delivered to theload, by the electromagnetic generator: $\begin{matrix}{P_{L} = {{T \cdot H_{C} \cdot W}\frac{\left( {Q_{bl} \cdot \alpha \cdot \omega_{n}} \right)^{2}}{8\left( {\frac{Q_{bl}}{\left\{ \frac{m}{T \cdot H_{C} \cdot W} \right\}\omega_{n}} + \frac{\rho}{\left\lbrack \frac{g \cdot B^{2}}{W} \right\rbrack}} \right)}}} & \left( {{Equation}\quad 2} \right)\end{matrix}$

where: P_(L) is the useful electrical power delivered to the load;

m is the total mass of the core shown in FIG. 7;

ω_(n) is the system natural circular frequency;.

B is the magnet flux density;

α is the peak amplitude excitation of the vibrations;

Q_(u) is the unwanted damping;

g is the gap length g in FIG. 7;

ρ is the resistivity of the coil in Ohm cm;

T is given by the depth T in FIG. 7;

H_(c) is given by l_(c) in FIG. 7; and

W is the length W in FIG. 7 (the magnetic core of the electromagneticgenerator would thereby fit within an enclosure (box) of length, W;height, H_(c); and depth, T).

The importance of the parameter ψ in Equation 1 for a core in agenerator design can be appreciated by examining Equation 2, wherein theterm in square brackets [. . . ] in Equation 2 is equal to the value ofthe variable ψ in Equation 1. The present inventors have therefore foundthat it is possible to maximise the value of the variable ψ by selectionof the geometry of the magnets and keeper plates of the magnetic corestructure shown in FIG. 7. This in turn allows the maximisation of theelectrical power which can usefully be provided by a vibration basedgenerator of the geometry shown in FIG. 7. The finding by the presentinventors that there is a geometrical configuration providing a maximumvalue for ψ, is not derivable from the prior art, and provides atechnical advantage over the prior art by providing a structure for anelectromagnetic generator which enables maximisation of the electricalpower output for a given set of external dimensions.

There are two other terms in Equation 2 that also depend on thegeometrical configuration in the expression for P_(L). The first is theamount of unwanted damping, represented by Q_(u); this will be acomplicated function of geometrical parameters, beam amplitude, andother factors such as details of the spring clamping at the beam root,that are not modelled in this analysis. It is acknowledged thatvariations in this parameter will have a significant effect on the poweroutput but this is not a parameter over which the designer has control.

The second is the term in curly brackets {. . . } that represents theaverage density of the core, excluding the mass of the coil. There maybe cases (especially at low excitation) when more power could begenerated by decreasing the gap, g. By doing this the electromagneticcoupling (and hence ψ) will be reduced, but the mass will be increased.However the ability of the designer to reduce this gap g is limited bythe size of the coil which must fit in this gap. This parameter istherefore not under control of the designer.

Therefore, for a given level of damping and coil size, the inventorshave found that maximising the variable ψ is the key to ensure maximumpower is generated in the load (i.e. maximum energy conversionefficiency).

Accordingly, as a result of examining the data-set from the batchprogram, it has been found by the present inventors that there exists asingle maximum for ψ in the three-dimensional parameter space ofg/t_(m), l_(m)/t_(m), l_(c)/t_(m).

The most preferred dimensional relationships, based on the maximum valuefor ψ, are listed in Table 1. TABLE 1 Parameter Value Error (%) W/l_(c)0.71 6 l_(m)/l_(c) 0.17 24 g/l_(c) 0.195 15 t_(m)/l_(c) 0.48 2t_(c)/l_(c) 0.087 12 Ψ= ψ_(max) 0.0491 0.5 Average B-field 0.366 11

The error associated with each of the entries in the table estimates theerror between the stated value, and the actual value of the parameter atthe maximum. The error is a result of numerical noise in the outputdata, which is caused by non-ideal element shapes in thinner areas ofthe model. This noise blurs the position of the maximum. It should benoted that ψ tends to decrease more slowly as g and lc are increasedfrom their most preferred value than if these quantities are decreased.Thus to ensure a good value of ψ in a design, it is better to err on theside of large g and l_(c).

In practice, the parametric ratios are encompassed within ranges, whichmay result from production tolerances, which preferably correspondapproximately to a ±10% variation in generator efficiency.

When the parameters are selected in accordance with the most preferreddimensional relationships as specified in Table 1, the magnetic fieldpattern for the magnetic core design is as shown in FIG. 12.

The parametric ratio t_(m)/l_(c) is of high importance in obtaining highefficiency because it relates to the magnet geometry. In accordance withthe invention, the parametric ratio t_(m)/l_(c) ranges from 0.40 to0.55, preferably from 0.43 to 0.53, and most preferably is about 0.48.

The parametric ratio l_(m)/l_(c) is of high importance in obtaining highefficiency because it also relates to the magnet geometry. In accordancewith the invention, the parametric ratio l_(m)/l_(c) preferably rangesfrom 0.1 to 0.24, and most preferably is about 0.17.

When the magnets are close together, i.e. with a low value of theparameter g, most of the flux lines flow straight across the gap, withlittle leakage. As the magnets are increasingly separated some of theflux curls around between magnets on the same side of the core. If themagnets are too close there is no space for the coil. In accordance withthe invention, the parametric ratio g/l_(c) preferably ranges from 0.14to 0.26, and most preferably is about 0.20.

The parametric ratio t_(c)/l_(c) is dependent on the thickness of theferromagnetic keeper which has a limited influence on efficiency. Inaccordance with the invention, the parametric ratio t_(c)/l_(c)preferably ranges from 0.06 to 0.12, and most preferably is about 0.09.

The parametric ratio W/l_(c) is dependent on the overall device width,which in turn is controlled by t_(c), l_(m) and g. In accordance withthe invention, the parametric ratio W/l_(c) preferably ranges from 0.61to 0.81, and most preferably is about 0.71.

The parameter ψ preferably has a value of from 0.04 to 0.06 Tesla², morepreferably a value of about 0.05 Tesla².

It should also be noted that if the dimensions determining the maximumsize of the electromechanical generator are not of the correctproportions to produce this optimum design, the optimum value can beapproached by splitting the available volume into several smallervolumes of a more ideal proportion.

1. An electromagnetic generator comprising a multilayer assembly of afirst layer carrying at least one magnet, a second layer carrying atleast one coil, and a third layer carrying at least one magnet, the atleast one magnet of the first and third layers being configured todefine therebetween a region of magnetic flux in which the at least onecoil is disposed, at least one of the layers being shaped to define arespective displaceable portion thereof which is displaceable byvibration of the electromagnetic generator thereby to cause relativemovement between the coil and the magnets and generate an electricalcurrent in the coil.
 2. An electromagnetic generator according to claim1 wherein the displaceable portion comprises an integral central body ofthe respective layer which is connected to a peripheral frame of therespective layer by an integral cantilever beam element.
 3. Anelectromagnetic generator according to claim 1 or claim 2 wherein eachmagnet comprises a layer applied on a surface of the first and thirdlayers respectively.
 4. An electromagnetic generator according to claim1 or claim 2 wherein each magnet comprises a magnetic body attached tothe respective layer.
 5. An electromagnetic generator according to claim4 wherein each magnet is located in a recess in the respective layer. 6.An electromagnetic generator according to any foregoing claim whereineach magnet is disposed on a surface of the first or third layerrespectively which faces the second layer.
 7. An electromagneticgenerator according to any foregoing claim wherein two magnets arecarried on each of the first and third layers, the two magnets of eachlayer presenting faces of opposite polarity towards the second layer. 8.An electromagnetic generator according to any foregoing claim whereinthe at least one coil comprises a layer applied on a surface of thesecond layer
 9. An electromagnetic generator according to claim 8wherein the at least one coil is a printed layer.
 10. An electromagneticgenerator according to any one of claims 1 to 7 wherein the at least onecoil is a wound coil attached to a surface of the second layer.
 11. Anelectromagnetic generator according to claim 10 wherein the wound coilis disposed in a recess formed in the surface of the second layer. 12.An electromagnetic generator according to any foregoing claim whereinthe at least one coil is disposed on a displaceable portion of thesecond layer.
 13. An electromagnetic generator according to anyforegoing claim further comprising at least one piezoelectric regionwhich is disposed on at least one of the layers and is adapted togenerate electrical current when the displaceable portion is displaced.14. An electromagnetic generator according to claim 13 wherein the atleast one piezoelectric region is disposed on a surface of thedisplaceable portion which is subjected to strain when the displaceableportion is displaced.
 15. An electromagnetic generator according toclaim 13 or claim 14 wherein the at least one piezoelectric region isdisposed on a surface of at least one of the layers which is subjectedto impact when the displaceable portion is displaced beyond a presetamplitude.
 16. A method of manufacturing an electromagnetic generator,the method comprising the steps of: (a) forming a first layer carryingat least one magnet, forming a second layer carrying at least one coiland forming a third layer carrying at least one magnet, at least one ofthe layers being shaped to define a respective displaceable portionthereof which is displaceable by vibration, the displaceable portioncarrying either the at least one magnet of the first and third layers orthe at least one coil of the second layer; and (b) assembling togetherthe first, second and third layers to form a multilayer structure inwhich the magnets of the first and third layers are configured to definetherebetween a region of magnetic flux in which the at least one coil isdisposed, the at least one displaceable portion being displaceable byvibration of the multilayer structure thereby to cause relative movementbetween the coil and the magnets and generate an electrical current inthe coil.
 17. A method according to claim 16 wherein in step (a) each ofthe first, second and third layers is formed as a part of a wafer, inwhich an array of a plurality of first, second and third layers,respectively, are formed.
 18. A method according to claim 17 wherein instep (b) wafers each having formed thereon the array of the plurality offirst, second and third layers, respectively, are assembled together toform a multilayer wafer assembly, and further comprising the step (c) ofcutting the multilayer wafer assembly into a plurality of individualmultilayer structures.
 19. A method according to claim 17 or claim 18wherein the displaceable portion comprises an integral central body ofthe respective layer which is connected to a peripheral frame of therespective layer by an integral cantilever beam element, and is formedby etching of the respective layer.
 20. A method according to any one ofclaims 17 to 19 wherein each magnet comprises a layer applied on asurface of the first and third layers respectively.
 21. A methodaccording to any one of claims 17 to 19 wherein each magnet comprises amagnetic body attached to the respective layer.
 22. A method accordingto claim 21 wherein each magnet is located in a recess in the respectivelayer.
 23. A method according to any one of claims 17 to 22 wherein eachmagnet is disposed on a surface of the first or third layer respectivelywhich faces the second layer.
 24. A method according to any one ofclaims 17 to 23 wherein two magnets are carried on each of the first andthird layers, the two magnets of each layer presenting faces of oppositepolarity towards the second layer.
 25. A method according to any one ofclaims 17 to 24 wherein the at least one coil comprises a layer appliedon a surface of the second layer
 26. A method according to claim 25wherein the at least one coil is a printed layer.
 27. A method accordingto any one of claims 17 to 24 wherein the at least one coil is a woundcoil attached to a surface of the second layer.
 28. A method accordingto claim 27 wherein the wound coil is disposed in a recess formed in thesurface of the second layer.
 29. A method according to any one of claims17 to 28 wherein the at least one coil is disposed on a displaceableportion of the second layer.
 30. A method according to any one of claims17 to 29 further comprising the step of applying at least onepiezoelectric region on at least one of the layers which is adapted togenerate electrical current when the displaceable portion is displaced.31. A method according to claim 30 wherein the at least onepiezoelectric region is disposed on a surface of the displaceableportion which is subjected to strain when the displaceable portion isdisplaced.
 32. A method according to claim 30 or claim 31 wherein the atleast one piezoelectric region is disposed on a surface of at least oneof the layers which is subjected to impact when the displaceable portionis displaced beyond a preset amplitude.
 33. An electromagnetic generatorcomprising at least two magnets and at least one coil disposedtherebetween, the at least two magnets being configured to definetherebetween a region of magnetic flux in which the at least one coil isdisposed whereby relative movement between the coil and the magnetsgenerates an electrical current in the coil, and at least onepiezoelectric region which is adapted to generate additional electricalcurrent by relative movement between the coil and the magnets.
 34. Anelectromagnetic generator according to claim 33 wherein at least one ofthe at least two magnets and at least one coil is carried on adisplaceable portion which is displaced by vibration to cause therelative movement between the coil and the magnets, and the at least onepiezoelectric region is disposed on a surface of the displaceableportion which is subjected to strain when the displaceable portion isdisplaced.
 35. An electromagnetic generator according to claim 33 orclaim 34 wherein at least one of the at least two magnets and at leastone coil is carried on a displaceable portion which is displaced byvibration to cause the relative movement between the coil and themagnets, and the at least one piezoelectric region is disposed on asurface of the electromagnetic generator which is subjected to impactwhen the displaceable portion is displaced beyond a preset amplitude.36. A magnetic core for an electromagnetic generator, the magnetic corecomprising four magnets disposed in two magnet pairs, with each pair ofmagnets being assembled with a respective keeper, the two pairs ofmagnets being mounted in an opposing manner so that a front end of eachmagnet of one magnet pair is spaced, in a first direction, from andfaces a front end of a corresponding magnet of the other magnet pair,the facing front ends being of opposite magnetic polarity, thereby todefine in the magnetic core a pair of gaps between the front ends of thefour magnets, and with rear ends of the magnets of each pair contactinga respective keeper, the magnets of each pair being mutually spaced in asecond direction, and wherein the ratio between the width of each magnetin the second direction to the height of the magnetic core in the seconddirection is from 0.40 to 0.55.
 37. A magnetic core according to claim36 wherein the ratio between the width of each magnet in the seconddirection to the height of the magnetic core in the second direction isfrom 0.43 to 0.53.
 38. A magnetic core according to claim 37 wherein theratio between the width of each magnet in the second direction to theheight of the magnetic core in the second direction is about 0.48.
 39. Amagnetic core according to any one of claims 36 to 38 wherein the ratiobetween the length of each magnet in the first direction to the heightof the magnetic core in the second direction is from 0.1 to 0.24.
 40. Amagnetic core according to claim 39 wherein the ratio between the lengthof each magnet in the first direction to the height of the magnetic corein the second direction is about 0.17.
 41. A magnetic core according toany one of claims 36 to 40 wherein the ratio between the length of eachgap in the first direction to the height of the magnetic core in thesecond direction is from 0.14 to 0.26.
 42. A magnetic core according toclaim 41 wherein the ratio between the length of each gap in the firstdirection to the height of the magnetic core in the second direction isabout 0.2.
 43. A magnetic core according to any one of claims 36 to 42wherein the ratio between the thickness of each keeper in the firstdirection to the height of the magnetic core in the second direction isfrom 0.06 to 0.12.
 44. A magnetic core according to claim 43 wherein theratio between the thickness of each keeper in the first direction to theheight of the magnetic core in the second direction is about 0.09.
 45. Amagnetic core according to any one of claims 36 to 44 wherein the ratiobetween the length of the magnetic core in the first direction to theheight of the magnetic core in the second direction is from 0.61 to0.81.
 46. A magnetic core according to claim 45 wherein the ratiobetween the length of the magnetic core in the first direction to theheight of the magnetic core in the second direction is about 0.71.
 47. Amagnetic core according to any one of claims 36 to 46 wherein theparameter ψ, which is defined by the equation${\psi = \frac{\int^{airgap}{B^{2}{\mathbb{d}A}}}{{total}\quad{area}\quad{of}\quad{core}}},$wherein B is the magnet flux density; and A is the total face area ofeach magnet pair of the core, the faces defining the gaps; and the totalarea of the core is the total face area of each magnet pair plus theface area of the gap therebetween, has a value of from 0.04 to 0.06Tesla².
 48. A magnetic core according to claim 47 wherein the parameterψ has a value of about 0.05 Tesla².
 49. A magnetic core according to anyone of claims 36 to 48 wherein the average magnetic field across thegaps is about 0.366 Tesla.
 50. An electromagnetic generator, theelectromagnetic generator comprising a magnetic core according to anyone of claims 36 to 49, a coil disposed in the pair of gaps and avibration sensitive mount for mounting one of the magnetic core and thecoil whereby vibration of the electromagnetic generator causes relativemovement of the magnetic core and the coil thereby to generate anelectrical current in the coil.
 51. A method of producing a magneticcore for an electromagnetic generator, the magnetic core comprising fourmagnets disposed in two magnet pairs, with each pair of magnets beingassembled with a respective keeper, the two pairs of magnets beingmounted in an opposing manner so that a front end of each magnet of onemagnet pair is spaced, in a first direction, from and faces a front endof a corresponding magnet of the other magnet pair, the facing frontends being of opposite magnetic polarity, thereby to define in themagnetic core a pair of gaps between the front ends of the four magnets,and with rear ends of the magnets of each pair contacting a respectivekeeper, the magnets of each pair being mutually spaced in a seconddirection, the method comprising the steps of: (a) establishing a modelfor the geometrical parameters of the magnetic core, the parametersincluding the width of each magnet in the second direction (t_(m)) theheight of the magnetic core in the second direction (l_(c)), the lengthof each magnet in the first direction (l_(m))and the length of the gapin the first direction (g); (b) varying the parameters to provide anoutput value ψ in units of Tesla² which is defined by the equation${\psi = \frac{\int^{airgap}{B^{2}{\mathbb{d}A}}}{{total}\quad{area}\quad{of}\quad{core}}};$wherein B is the magnet flux density; and A is the total face area ofeach magnet pair of the core, the faces defining the air gaps; and thetotal area of the core is the total face area of each magnet pair plusthe face area of the gap therebetween; (c) determining a maximum for theparameter ψ; (d) determining values of at least the parameters (t_(m)),(l_(c)), (l_(m)) and (g) to provide a range for the parameter ψ whichencompasses the maximum for the parameter ψ; and (e) producing themagnetic core having the determined values of the parameters (t_(m)),(l_(c)), (l_(m)) and (g) within a particular tolerance.
 52. A methodaccording to claim 51 wherein the ratio between the width of each magnetin the second direction (t_(m)) to the height of the magnetic core inthe second direction (l_(c)) is from 0.40 to 0.55.
 53. A methodaccording to claim 52 wherein the ratio between the width of each magnetin the second direction (t_(m)) to the height of the magnetic core inthe second direction (l_(c)) is from 0.43 to 0.53.
 54. A methodaccording to claim 53 wherein the ratio between the width of each magnetin the second direction (t_(m)) to the height of the magnetic core inthe second direction (l_(c)) is about 0.48.
 55. A method according toany one of claims 51 to 54 wherein the ratio between the length of eachmagnet in the first direction (l_(m)) to the height of the magnetic corein the second direction (l_(c)) is from 0.1 to 0.24.
 56. A methodaccording to claim 55 wherein the ratio between the length of eachmagnet in the first direction (l_(m)) to the height of the magnetic corein the second direction (l_(c)) is about 0.17.
 57. A method according toany one of claims 51 to 56 wherein the ratio between the length of eachgap (g) in the first direction to the height of the magnetic core in thesecond direction (l_(c)) is from 0.14 to 0.26.
 58. A method according toclaim 57 wherein the ratio between the length of each gap (g) in thefirst direction to the height of the magnetic core in the seconddirection (l_(c)) is about 0.20.
 59. A method according to any one ofclaims 51 to 58 wherein the ratio between the thickness of each keeperin the first direction (t_(c)) to the height of the magnetic core in thesecond direction (l_(c)) is from 0.06 to 0.12.
 60. A method according toclaim 59 wherein the ratio between the thickness of each keeper in thefirst direction (t_(c)) to the height of the magnetic core in the seconddirection (l_(c)) is about 0.09.
 61. A health and usage monitoringsystem (HUMS) for an aircraft, the system incorporating at least oneelectromagnetic generator according to claim 1, claim 33 or claim 50.62. A health and usage monitoring system (HUMS) for an aircraftaccording to claim 61, the system including a sensor and a local wireless transmission system, both the sensor and the wire less transmissionsystem being powered by the electromagnetic generator.
 63. A sensingsystem for railway lines and associated components, the systemincorporating at least one electromagnetic generator according to claim1, claim 33 or claim
 50. 64. A sensing system for railway lines andassociated components according to claim 63, wherein the electromagneticgenerator is adapted to generate power from the vibration provided bythe passage of a train, either directly from the rail line or via acantilever attached to the rail line.
 65. A sensing system for railwaylines and associated components according to claim 64, wherein thesensing system includes a sensor and means to telemeter the output datato a remote location.
 66. A vehicle battery charger system incorporatingat least one electromagnetic generator according to claim 1, claim 33 orclaim
 50. 67. A vehicle battery charger system according to claim 66incorporated into a battery recharging system for a tracking system fora lorry or truck trailer.
 68. A mobile telecommunications equipmentincorporating at least one electromagnetic generator according to claim1, claim 33 or claim
 50. 69. A conditioning monitoring systemincorporating at least one electromagnetic generator according to claim1, claim 33 or claim
 50. 70. An electromagnetic generator substantiallyas hereinbefore described with reference to the accompanying drawings.71. A method of manufacturing an electromagnetic generator substantiallyas hereinbefore described with reference to the accompanying drawings.72. A magnetic core for an electromagnetic generator substantially ashereinbefore described with reference to the accompanying drawings. 73.A method of producing a magnetic core for an electromagnetic generatorsubstantially as hereinbefore described with reference to theaccompanying drawings.